$configFile = 'test.xml'
$xml = [xml](Get-Content -Path $configFile -Encoding Ascii)
foreach($button in $xml.SelectNodes('/config/button')) {
$button.command = $button.command.Replace('C:\Prog\Laun.jar', 'C:\Prog32\folder\test.jar')
}
$xml.InnerXml | Out-File -FilePath $configFile -Encoding ascii